In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the complexity of the electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles. Such alternative fuel vehicles typically use an electric motor, perhaps in combination with another actuator, to drive the wheels.
Discontinuous Pulse Width Modulation (DPWM) methods are often employed for controlling the switching action of three phase voltage source inverters which are used to control the phase currents of three-phase alternating current (AC) motors. A significant advantage of DPWM methods over continuous PWM methods, such as sinusoidal or space vector modulation, is reduced inverter switching losses, which helps to improve the efficiency of hybrid electric vehicles, particularly when only the electric motors are in use. DWPM methods differ from continuous PWM methods in that only one zero vector is used in a given switching cycle. As a result, each switch in a three phase inverter is typically not switched for 60° segments of an electrical cycle. The location of the 60° clamped segment with respect to the inverter output voltage and load power factor determines the type of DPWM method and resulting PWM properties.
Ideally, the switch pairs in each phase leg of the three-phase voltage source inverter each operate in a complimentary fashion such that one switch is always “on” and the other switch is always “off.” In practice, however, a blanking time, or dead-time, is typically inserted during each transition of a switching state of the voltage source inverter. The dead-time is a short interval during which both switches are gated “off.” This prevents both switches in a phase leg of the voltage source inverter from simultaneously being “on,” which could short-circuit the voltage source inverter.
Additionally, the gate drive circuitry may have limitations or the switches may impose limitations on the minimum “on” time duration that is commanded (e.g., directed by a control module, processor, or the like) to a switch in the voltage source inverter. The minimum pulse width and dead-time limitations result in finite minimum (e.g., non-zero) and maximum (e.g., non-unity) values of duty cycle which can be commanded by the controller (e.g., a DPWM modulator).
These non-linear effects, dead-time and minimum pulse width, introduce distortion on ideal inverter output voltages as produced by DPWM control. Since DPWM control offers reduced losses compared to continuous PWM methods, it is desirable to employ DPWM control methods while simultaneously minimizing the distortion caused by the non-linear inverter effects. Minimizing distortion (which may increase torque ripple) is particularly important in hybrid vehicles which can be operated modes where the vehicle is driven via only electric power. Various compensation methods have recently been developed to reduce the distortion effects of inverter non-linearities on DPWM control. However, the various compensation methods have not been employed under a single, unitary control method.
Accordingly, it is desirable to provide a control method that employs the DPWM compensation method most suitable for the current system operating conditions. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.